A method is referred to in Published Patent Application No. DE 199 58 406 A1, which describes a piezoactuator that is used, for example, in a fuel injector. The piezoactuator behaves similarly to a capacitative element in electrical terms, and is therefore itself often referred to as a capacitative element. The capacitative element is longer or shorter depending on its charge state. The change in length of the capacitative element is transferred to a valve element of the fuel injector.
In the event of an interruption in activation of the capacitative element, or a malfunction of one of the components, it may happen that the capacitative positioner remains continuously in one specific position because it can no longer be discharged. In a context of use in a fuel injector, the result of this may be, for example, that the latter remains in the open position for a long period of time, and fuel is continuously injected into the combustion chamber of the internal combustion engine. This may result in severe damage to the internal combustion engine.
The ohmic resistance is provided to prevent such a situation. It enables discharging of the capacitative element even when the actual control line is interrupted, e.g. due to a cable break or a contact fault. The value of the ohmic resistance is dimensioned such that the time constant resulting from the capacitative element and the ohmic resistance is so great that no significant discharge of the capacitative element occurs within the usual activation time period that is usual for fault-free injection. On the other hand, the time constant is set so that the capacitative element is sufficiently discharged within the maximum time available before the valve must definitely be closed in order not to damage the internal combustion engine.
German Published Patent Application No. 199 58 406 proposes to sense the value of the ohmic resistance at specific points in time, and to draw conclusions therefrom as to the nature and/or the temperature of the capacitative element. The temperature dependence of the capacitative element may thereby be corrected.